Endoscopes have become widely used in the medical and industrial fields. A capsule endoscope that integrates the imaging optical system into a capsule that can be swallowed has been used in the medical field. For instance, Japanese Laid-Open Patent Application 2003-260024 discloses one example of a capsule endoscope that includes an objective lens for forming an image of an object, a solid-state image sensor for converting the optical image into an electronic signal, and a light emitting diode for illuminating the object, all of which are integrated within a hemispherical or nearly hemispherical transparent cover.
Additionally, Japanese Laid-Open Patent Application 2003-5031 discloses an example in which a single aspheric lens element is used in the objective optical system so that production costs can be reduced and the imaging optical system can be miniaturized, which in turn generally enables producing a small electronic imaging device suitable for devices such as a cellular phone.
An imaging optical system that captures a good quality image that is small and suitable for viewing with an endoscope, and that also can be produced at a low cost, is demanded even in an endoscope wherein the imaging optical system is integrated into a hemispherical or nearly hemispherical transparent cover at the tip of, for example, a capsule endoscope. Unfortunately, however, if the imaging optical system described in Japanese Laid-Open Patent Application 2003-5031 were to be applied to this type of endoscope, a good quality image that is suitable for viewing with an endoscope would not be obtained although a reduction in cost could be achieved. The imaging optical system described in Japanese Laid-Open Patent Application 2003-5031 is designed as an optical system for capturing images of objects generally in an outdoor setting.